Crack growth in a coated gas turbine superalloy under thermo-mechanical fatigue

The out-of-phase thermo-mechanical fatigue (TMF) behaviour of the single crystal superalloy CMSX-4 was studied in the uncoated and in the low-pressure plasma sprayed (LPPS) coated condition. In addition, growth measurements of in-depth coating cracks were performed. Cyclic hardening of the substrate and superimposed multiple crack initiation/propagation within the coating leads to cyclic softening that consumes a significant portion of TMF lifetime. A three-step crack propagation mode of the coated material is described. Special attention is paid to the role of the inter-diffusion zone, since coating cracks are decelerated in the inter-diffusion zone before penetrating into the substrate.